1. Field of the Invention
The present invention relates to an apparatus for directly measuring insulation thermal performance at cryogenic temperatures. More particularly, it relates to such an apparatus that facilitates accurate performance determinations without measuring or quantifying cryogenic material boil-off.
2. Description of Related Art
The ability to store large amounts of cryogenic fluids for long durations will have a profound effect on the success of many future space programs using propellant, reactant, and life support cryogens. These missions will require on-orbit systems capable of long-term storage of cryogens for applications such as: space platforms using electric propulsion, space-based lasers, orbit-transfer vehicles, and orbital-propellant depots. The high cost of delivering payload mass to orbit will require storage systems capable of significantly limiting cryogenic losses due to boil-off, particularly for mission durations of up to ten years or longer.
High performance insulation systems are essential to meet these low boil-off requirements. For missions that require zero or near-zero boil-off, the performance of passive insulation systems must be maximized in order to minimize the power requirement of an active refrigeration system. Although Multi-Layer Insulation (MLI) systems have been extensively used to insulate cryogenic vessels in a space environment, it has been for short-duration missions that require from 30 to 50 layers to meet the mission requirements. Conversely, 150 layers or more of MLI will likely be needed to meet the requirements of future long-term missions. Limited data exists on the performance and physical characteristics of these thick MLI systems.
Optimization of the passive insulation systems using thick MLI will be required to minimize the mass, volume, and power consumption of any active cooling technologies. Current passive insulation systems are unable to meet the thermal performance needs of future space missions requiring long-term storage of cryogenic fluids. Critical information on the performance of thick MLI systems—a fundamental requirement—is lacking. Therefore, systematic investigation of thick MLI performance at liquid hydrogen temperatures would provide an opportunity to quantify vitally needed data for future missions. The ability to test with liquid hydrogen (LH2) is desirable, because LH2 represents the coldest cryogenic liquid planned for most missions and would, therefore, verify thick MLI performance for the most challenging applications.
Disclosed herein is an apparatus that measures insulation thermal performance, including thick MLI systems, that does not rely on measurement of cryogenic liquid boil-off to calculate insulation thermal performance, thereby avoiding the inherent difficulties to a) eliminate ambient heat leak into a cryogenic tank that bypasses the insulation (which will introduce errors in the heat flux calculations), and b) precisely measure very low boil-off flow rates.